Since a cold work tool is used in contact with a hard workpiece, the tool is required to have a sufficient hardness to resist the contact. Conventionally, alloy tool steels of the SKD10 or SKD11 series for example, which are JIS steel grades, have been used for cold work tool materials (see Non Patent Literature 1). Furthermore, an alloy tool steel having an improved composition from the above alloy tool steels has been proposed in response to demands for further increased hardness (see Patent Literature 1).
Typically, a cold work tool material is manufactured from a raw material, as a starting material, such as a steel ingot or a bloom which is produced from the ingot. The starting material is subjected to various hot working and heat treatment to form a predetermined steel material, and then the steel material is subjected to an annealing process to produce the cold work tool material. The cold work tool material in the annealed condition having a low hardness is typically supplied to a manufacturer of a cold work tool. The material is machined into a shape of the tool, and thereafter quenched and tempered to adjust its hardness for use. After the adjustment of the hardness, finishing machining is typically conducted. In some cases, quenching and tempering are conducted first for the material in the annealed condition, and then the machining is conducted for the tool shaping together with the finishing machining. Here, the term “quenching” refers to an operation where a cold work tool material (or a cold work tool material that has been subjected to machining) is heated in an austenitic phase temperature range and then rapidly cooled to transform it into a martensitic structure. Thus, the cold work tool material has such a composition that can have a martensitic structure by the quenching.
In this connection, it has been known that a hardness of a cold work tool can be improved by controlling a martensitic structure after quenched. For example, techniques for adjusting an amount of retained austenite in a matrix after quenched (see Patent Literature 2), and techniques for adjusting an amount of chromium or molybdenum dissolved in the matrix after quenched (see Patent Literatures 3 and 4) were proposed.